This study will test how well Fenoldopam works in lowering blood pressure in patients experiencing a hypertensive emergency. For this protocol, hypertensive emergencies will be defined as a supine diastolic blood pressure 3 120 mmHg with evidence of acute or chronic end-organ damage or supine diastolic blood pressure 3 140 mmHg without evidence of end-organ damage. The rationale for the clinical use of intravenous Fenoldopam is based on its potent systemic vasodilatory properties which suggest that it may be effective in the acute lowering of blood pressure. The objectives of this double-blind (with respect to dose), randomized, multicenter study are: 1) to evaluate the antihypertensive safety and efficacy of various doses of intravenous Fenoldopam (0.01, 0.03, 0.1 or 0.3 ug/kg/min) over a 24-hour period, with the strongest possible attempt to maintain a constant rate of infusion for at least the first 4 hours and, ideally, over an entire 24-hour period; 2) to determine the plasma and urine levels of Fenoldpam and various metabolites during the Fenoldopam infusion and the post-infusion washout period.